Various freeze processes have been developed to produce potable water from seawater or brackish water; to concentrate fruit juices such as orange juice and grape juice, vegetable juices such as tomato juice, and coffee; and to separate dissolved or suspended salts form the liquid carrier. See, for example, the U.S. Pat. Nos. of Ashley et al 3,070,969; Ashley 3,477,241; Ashley 3,501,924; Ganiaris 3,620,034; Johnson 3,664,145; and Ogman 4,091,635.
Many types of equipment and heat exchangers have been used in the described freeze processes, but shell and tube heat exchangers, although widely used for heat exchange, apparently have been used only on a limited basis as freeze exchangers.
A shell and tube heat exchanger has an array of tubes extending between and through two spaced apart tube sheets surrounded by a shell. The shell is provided with an inlet and an outlet so that a suitable heat exchange fluid can be circulated through the shell to cool or heat a fluid flowing through each tube.
Each end of the array of tubes can be left open, or exposed, for use in some processing operations. For other operations, one or both ends can be enclosed by a fluid retaining header, which may or may not have a removable cover or access port. When only one header is present it can be either a fluid inlet or fluid outlet header. When a header is positioned at each end, one header can be a fluid inlet while the other can be a fluid outlet. Such an arrangement is conventional for once-through or single pass heat exchangers. The fluid inlet and outlet headers, or portions thereof, are provided with suitable conduit means for supplying and removing fluid.
Although shell and tube heat exchangers are generally used to heat a fluid stream, they can be used for cooling such a stream. Shell and tube heat exchangers of the described types can be used as freeze exchangers for producing fresh water from brackish water and seawater, for concentrating fruit and vegetable juices, and in industrial crystallization processes. As the liquid flows through each tube, it can be cooled enough to crystallize a solid from the liquid. Thus, by cooling seawater, ice is obtained which when separated, washed, and melted provides potable water. When a fruit or vegetable juice is similarly chilled, ice forms and is removed to provide a concentrated juice.
Heat exchangers of the described types can use any cooling fluid on the shell side to cool a liquid flowing through the tubes. The fluid can be fed through one end and removed through the other end of the heat exchanger in a substantially unidirectional flow. Some suitable cooling fluids are ammonia and Freon brand refrigerants.
One of the major problems in freeze concentrating a liquid mixture in a shell and tube freeze exchanger has been the deposition of frozen solvent (usually ice) or solute, on the freeze exchanger surfaces. Buildup of a solid, such as ice, lowers heat transfer and reduces the efficiency of the apparatus. In addition, if buildup of ice or some other frozen solvent or solid continues it can plug the apparatus completely, making it necessary to shut down to thaw the frozen material. This can happen very readily because the solids usually first deposit at the inlet ends of the tubes and on the adjacent tube sheet. Accordingly, a need exists for improved shell and tube freeze exchangers which can be used over extended time periods by reducing or avoiding formation of solids deposited on the inlet tube ends and tube sheet. Also needed is apparatus for freeze concentrating liquid mixtures using shell and tube freeze exchangers which avoids or minimizes formation of solid deposits.